Structs, properties, and methods | Swift

A struct is Swift’s default tool for modeling data. It groups related values and behavior into one type.

Stored properties

Stored properties hold data:

struct Article {
    let id: String
    let title: String
    var isFavorite: Bool
}

let article = Article(id: "a-1", title: "Swift basics", isFavorite: false)

Use let for properties that should not change after creation. Use var only when mutation is part of the model.

Computed properties

A computed property calculates a value instead of storing it:

struct ReadingProgress {
    let completedPages: Int
    let totalPages: Int

    var percentage: Double {
        Double(completedPages) / Double(totalPages)
    }
}

Computed properties are useful for values derived from other stored data. They avoid storing duplicated state that can drift out of sync.

Methods

Methods are functions attached to a type:

struct Cart {
    var items: [String]

    func contains(_ item: String) -> Bool {
        items.contains(item)
    }
}

If a method changes a struct, mark it as mutating:

struct Counter {
    private(set) var value = 0

    mutating func increment() {
        value += 1
    }
}

private(set) means code outside the struct can read value but cannot set it directly. This keeps mutation behind a clear method.

Static members

Static members belong to the type itself, not an instance:

struct AppConfig {
    static let supportEmail = "support@example.com"
}

Use static constants for shared facts that do not depend on one instance.

What to carry forward

structs are Swift’s default modeling tool
stored properties hold data
computed properties derive data
methods attach behavior to data
mutating marks methods that change a struct
static members belong to the type

Next, you will compare structs with classes.